Ultrasonic inspection method for inaccessible pipe and tubing



Nov. 23, 1965 l. L. JOY

uLTRAsoNIc FLAw TESTING APPARATUS 2 Sheets-Sheet 1 Filed Oct. 26. 1960WMM jizz/672 far' Iva/2z L.

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ULTRASONIC FLAW TESTING APPARATUS 2 Sheets-Sheet 2 Filed Oct. 26. 1960United States Patent O 3,218,845 ULTRASONIC INSPECTION METHOD FORINACCESSIBLE PIPE AND TUBING Daniel C. Worlton, Richland, Wash.,assignor to the United States of America as represented by the UnitedStates Atomic Energy Commission Filed July 9, 1962, Ser. No. 208,655 2Claims. (Cl. 73-67.8)

This invention relates to methods of nondestructive testing `and moreparticularly to methods using Lamb waves to detect subsurface aws inmetal pipe. The invention described herein was made in the course of, orunder, a contract with the United States Atomic Energy Commission.

Conventional ultrasonic inspection of metal pipe is accomplished using awater coupling medium between a transducer and the metal pipe. The metalpipe is immersed in the water and complete inspection is accomplished byrotating the pipe as the transducer is moved along the longitudinal axisthereof. In some cases, inspection is made using a shoe as the couplingmedium between the transducer and the pipe; the shoe being movedcircumferentially around the pipe and along the longitudinal axisthereof to accomplish complete inspection. Where, however, the tubes areinstalled and there is only limited access to the surface thereof, theabove described techniques are inoperable.

It is therefore one object of the present invention to provide a methodfor nondestructively detecting subsurface flaws in a metal pipe whereinonly a limited portion of said pipe is available for testing.

It is another object of the present invention to provide a method usingLamb waves for detecting aws in a metal pipe wherein only a smallportion of said pipe is available for testing.

It is another object of the present invention to provide a method fordetecting flaws in a metal pipe using Lamb waves which travel the lengthof said pipe in a helical pattern.

Other objects of the present invention will become more apparent as thedetailed description proceeds.

In general, the present invention comprises generating ultrasonic planewaves and causing them to impinge on the surface of a metal pipe at apredetermined angle of incidence in a first plane passing through thelongitudinal axis of the pipe. The ultrasonic waves are also caused toimpinge on the surface of the pipe at an angle of incidence in a secondplane passing through the longitudinal axis of the pipe normal to saidfirst plane. The ultrasonic waves cause Lamb waves to be excited in thepipe and ow therealong in a helical pattern. Any ilaws in the pipe causereflections of the Lamb waves, which reflections return along thehelical pattern to be detected and thereby denote the presence of flawsin the pipe.

Further understanding of the present invention will best be obtainedfrom consideration of the accompanying drawings in which:

FIG. l is a graphical representation of Lamb wave modes in stainlesssteel plate as a function of phase velocity versus the product offrequency and thickness of the plate.

FIG. 2 is a sketch illustrating the method of propagation of the Lambwaves for the present invention.

FIG. 3 is a top view of the sketch of FIG. 2.

When an ultrasonic wave impinges on a metal plate, an infinite number ofLamb wave modes may be generated within the plate depending upon thematerial of the plate, the thickness of the plate and the frequency ofthe ultrasonic wave. Each Lamb wave mode is propagated with a uniquephase velocity that is related to the product of the frequency of theultrasonic wave and of thickness of 3,218,845 Patented Nov. 23, 1965 ICCthe plate. FIG. 1 illustrates this relationship for stainless steelplate. In FIG. l, curves 10, 12, 14, 16, 18 and 20 are plots of the 1st,2nd, 3rd, 4th, 5th and 6th asymmetrical modes respectively. Thus, wherethe thickness of the plate is d1 and an ultrasonic wave of frequency f1is applied to the plate, the product fldl will permit the excitation ofthe rst, second or third Lamb Wave modes in the plate. The correspondingphase velocities of the excited Lamb waves will be V1, V2 or V3 as shownin FIG. 1.

The point of contact between a plane wave front and the plate runs alongthe surface of the plate with a veloci ity V which is equal to VW/sin 0,where VW is the velocity of the ultrasound in the coupling medium and 0is the angle of incidence of the ultrasonic wave. When 0 is chosen sothat the surface velocity V is equal to the phase velocity of aparticular Lamb wave mode then that particular mode will be excited inthe plate provided that the frequency times thickness therefor iscorrect. The above described method of Lamb wave mode propagation in ametal plate may be directly applied to metal pipe. That is, thecurvature of the pipe does not appreciably alter the behavior of modepropagation from that of plate geometries.

Turning to FIGURES 2 and 3, a transducer 22 is mounted on a wedge-shapedshoe 24, which in turn is mounted on the surface of a metal pipe 26. Theshoe 24 is composed of some suitable ultrasonic coupling medium such asplastic. The shape of the shoe 24 is cut at an angle 0 with respect tothe surface of the pipe 26 so that ultrasonic plane waves emanating fromtransducer 22 will irnpinge on the surface of the metal pipe 26 toexcite a particular Lamb wave mode therein in accordance with the curvesillustrated in FIG. 1. The particular mode to u be established Withinthe pipe 26 depends upon the application. For example, suppose that itis desirable that the second asymmetrical mode be excited in a stainlesssteel pipe whose Wall thickness is 1%. First, a test frequency is chosenaccording to defect, resolution requirements, and wave attenuationcharacteristics. For purposes of illustration, assume an ultrasonic wavepropagation frequency of 2.5 megacycles. FIG. l shows that the secondasymmetrical mode of this frequency will propagate in the chosen pipe ata phase velocity of 2.75 inches per second. The transducer wedge angle 0is computed from the prior described equation,

The plastic shoe 24 has a VW value of about 0.9 l05 inches per second.0, therefore, would be 19 degrees.

By rotating the transducer 22 through an angle with respect to thelongitudinal axis of the pipe 26, the ultrasonic waves emanating fromtransducer 22 will be caused to impinge on the surface of the pipe 26 ata similar angle with respect to the longitudinal axis of the pipe 26.This angle gives a transverse direction to the Lamb waves therebycausing them to travel down the pipe in a helical pattern as shown. Thegreater the angle the smaller the pitch of the helices and, in general,the smaller the pitch the greater the resolution to small defects.However, there is a practical limitation as to how small the pitch maybe, since the smaller the pitch the less the longitudinal travel.

As the Lamb waves travel down the pipe 26 in a helical path, they may,in the absence of flaws in the pipe 26, do one of two things. If thepipe 26 has a small enough length, the Lamb waves will reach the end ofthe pipe 26 whereupon they will be reected back along the same helicalpath as they have previously traveled and Will be detected by transducer22. If the pipe is infinitely long the Lamb waves Will be attenuated andno reflected sigsin 0= nal will be received. If a flaw exists in thepipe 26,4 it will cause the Lamb waves to be reflected, whichreflections will travel back along the helical pattern and will bedetected by transducer 22. Thus, llaws in the pipe 26 may be detected bythe time difference in reflected signals detected by transducer 22 wherethe length of pipe26 is short and by the presence of. reected signalswhere the pipe 26 is long. The actual recognition of these signals maybe accomplished by commercially available equipment such as aReectorscope manufactured by Sperry Products, Inc.

Complete scanning of the `pipe may be accomplished simply by (a) varyingthe angle of the transducer 22, or (b) moving the transducer 22 back andforth slightly along the longitudinal axis of the pipe 26 whilemaintaining the angle constant, or (c) mounting several transducers in arow.

Persons skilled in the art will, of course, readily adapt the teachingsof the invention t methods far different from the methods hereindescribed` Accordingly, the scope of the protection afforded theinvention should not be limited to the particular methods described andshown above but should be determined only in accordance with theappended claims.

What is claimed is:

1. A method of detecting ilaws in a metal pipe cornprising spatiallymounting a transducer at a predetermined angle with respect to thesurface of said pipe in a first plane passing through the longitudinalaxis thereof, interposing a coupling medium between said transducer andsaid pipe, positioning said transducer at an angle with respect to thelongitudinal axis of said pipe in a second plane passing through saidlongitudinal axis normal to said first plane, moving said transducerback and forth a short distance along the longitudinal axis of said pipewhile holding the angular position of said transducer constant, excitingsaid transducer to generate ultrasonic plane waves therefrom, saidultrasonic waves impinging on the surface of said pipe and causing LambWaves to be excited therein which travel along said pipe in a helicalpath, said aws in said pipe causing reections of said Lamb waves, anddetecting said reilected waves, thereby detecting the presence of flawsin said pipe.

2.A method of detecting flaws in a metal pipe comprising spatiallymounting a transducer at a predetermined angle with respect to thesurface of said pipe in a first plane passing through the longitudinalaxis thereof, interposing a coupling medium between said transducer andsaid pipe, positioning said transducer at an angle with .respect to thelongitudinal laxis of said pipe in a second plane passing through saidlongitudinal axis normal to said first plane, varying the angle of thetransducer With respect to the longitudinal axis in said second plane,exciting said transducer to generate ultrasonic plane waves therefrom,said ultrasonic waves impinging on the surface of said pipe and causingLamb waves to be excited therein which travel along said pipe in ahelical path, said aws in said pipe causing reflections of said Lambwaves, and detecting said reflected waves, thereby detecting thepresence of flaws in said pipe.

References Cited by the Examiner UNITED STATES PATENTS 2,536,128 1/1951Firestone et al. 73-67.8

FOREIGN PATENTS 756,906 1/ 1957 Great Britain.

OTHER REFERENCES McMasters: Nondestructive Testing Handbook, Section 45,pages 12-13 and Section 47, pages 26-28, published 1959 by the RonaldPress Company of New York.

RICHARD C. QUEISSER, Primary Examiner.

JOHN P. BEAUCHAMP, Examiner.

1. A METHOD OF DETECTING FLAWS IN A METAL PIPE COMPRISING SPATIALLYMOUNTING A TRANSDUCER AT A PREDETERMINED ANGLE WITH RESPECT TO THESURFACE OF SAID PIPE IN A FIRST PLANE PASSING THROUGH THE LONGITUDINALAXIS THEREOF, INTERPOSING A COUPLING MEDIUM BETWEEN SAID TRANSDUCER ANDSAID PIPE, POSITIONING SAID TRANSDUCER AT AN ANGLE WITH RESPECT TO THELONGITUDINAL AXIS OF SAID PIPE IN A SECOND PLATE PASSING THROUGH SAIDLONGITUDINAL AXIS NORMAL TO SAID FIRST PLANE, MOVING SAID TRANSDUCERBACK AND FORTH A SHORT DISTANCE ALONG THE LONGITUDINAL AXIS OF SAID PIPEWHILE HOLDING THE ANGULAR POSITION OF SAID TRANSDUCER CONSTANT, EXCITINGSAID TRANSDUCER TO GENERATE ULTRASONIC PLANE WAVES THEREFROM, SAIDULTRASONIC WAVES IMPINGING ON THE SURFACE OF SAID PIPE AND CAUSING LAMBWAVES TO BE EXCITED THEREIN WHICH TRAVEL ALONG SAID PIPE IN A HELICALPATH, SAID FLAWS IN SAID PIPE CAUSING REFLECTIONS OF SAID LAMB WAVES,AND DETECTING SAID REFLECTED WAVES, THEREBY DETECTING THE PRESENCE OFFLAWS IN SAID PIPE.